The Stop Knot is commonly used by match anglers who need to floatfish a waggler in water much deeper than the length of their rod, but it is even more commonly used by predator anglers when they create their float rigs.
A well-tied Stop Knot provides a moveable point that helps prevent your float from sliding all the way along your mainline, and if it is tied using a length of flexible mono, it will cast through the rings of any match rod on the market.
Specialist anglers tend to use powergum to tie their stop knots, which will still pass through their rod's rings because of the large internal diameter of them.
Lay a 6in length of powergum or thick mono alongside your mainline.
Form a loop with the powergum or mono and thread one end through the loop and over your mainline.
Repeat threading the powergum or thicker mainline through the loop and around your mainline four or five times.
Moisten the knot with saliva, slide it into position upon the mainline then pull it tight. Trim the tag ends to around an inch from the knot.